One field of application for the disclosed bone anchoring device is holding together a bone, e.g. a vertebra, and a support member, e.g. a rod, in a desired relative position, while allowing in some cases a limited amount of relative movement, for example to aid in healing of breaks or to correct bony structure deficiencies and abnormalities. In particular, sufferers of abnormal spine curvature or other spine deficiencies may benefit from the disclosed bone anchoring device.
When the spine of a person has abnormal curvature or other deficiencies, e.g. a damaged intervertebral disc, vertebrae usually need to be stabilized in a correct position relative to one another.
A common technique for providing vertebral stabilization is to fix bone anchors, such as pedicle screws or hooks, to certain vertebrae, the bone anchors being then, in turn, fixed to a support member, usually an elongate structure such as a rod. The rod connects the anchors together, thereby providing stabilization between the vertebrae.
U.S. Pat. No. 5,005,562 discloses a bone screw having a threaded shank to be anchored into the bone and a fixed head formed integrally with the shank, the fixed head having a U-shaped slot for receiving a rod. This so-called monoaxial bone screw has a high resistance to loads. However, the screw head cannot be moved relative to the shank and the rod or the bone screw must be favorably positioned in order for the rod to be placed within the U-shaped slot. This may be sometimes very difficult to do.
For this reason, polyaxial bone screws may be preferred. Polyaxial bone screws allow rotation of the head, or receiver, about the threaded shank until a desired rotational position of the receiver is achieved relative to the shank. Thereafter, a rod can be inserted into the receiver and eventually the receiver is locked or fixed in a particular position relative to the threaded shank.
U.S. Pat. No. 7,875,065 discloses an example of a polyaxial bone screw. This screw comprises a shank having a threaded portion topped by a shank head, a receiver for receiving the shank head and a retainer structure comprising two discrete pieces. The retainer pieces totally circumferentially surround the shank head and are captured between the receiver and the shank head. As shown in FIGS. 6 to 8 of U.S. Pat. No. 7,875,065, for assembling the bone screw, the retainer pieces are first top-loaded into the receiver, passed therethrough and placed in the bottom thereof. Then, the shank head is bottom-loaded into the receiver, between the two retainer pieces. However, placing the retainer pieces in their right position may present practical difficulties and take time. Moreover, misplacement of the retainer pieces may make the shank head difficult to insert between them. Furthermore, in the disclosed embodiments, the shank head is non-spherical and articulation occurs between the retainer pieces and the receiver.
While polyaxial bone anchoring devices and, in particular, polyaxial bone screws have proven effective, further improvements, especially regarding their assembly and handling, would be beneficial.